The present invention relates to a strobe light and more particularly to a high intensity strobe light for anti-collision systems.
An increased usage of strobe lights on aircraft, particularly military aircraft, is being made so that a high percentage of mid-air collisions and near-misses might be eliminated. A recent safety survey indicates that about 85 percent of mid-air collisions and near-misses have occurred during daylight with clear weather conditions and with about 80 percent of the aircraft having radar and/or radio contact with air traffic control at the time. These statistics tend to confirm that the present day incandescent rotating beacons on aircraft are adequate for night operation but are not bright enough for daylight operation in clear weather.
The replacement of rotating beacons with presently available strobe lights has caused additional problems with the main problem being that of electromagnetic interference. Commercially available strobe lights generally have a zenon flash tube and reflector, a bank of capacitors, and power supply and timing circuit, all of which are encased in a housing and mounted outboard on an aircraft appendage, such as a wing or tail. These strobe lights, which might flash 60 to 70 times a minute, produce electromagnetic radiation which interfers with the aircraft's navigation and communication system to such an extent that the safety of the aircraft becomes of concern.
A flash tube is generally comprised of two spaced apart electrodes within a sealed glass envelope having a rare gas fill, typically xenon, at a sub-atmospheric pressure. Such lamps are connected across a large capacitor charged to a substantial potential, which is, however, insufficient to ionize the xenon fill gas. Upon application of an additional pulse of sufficient voltage, the xenon is ionized, and an electric arc is formed between the two electrodes, discharging the large capacitor through the flash tube, which emits a burst of intense light, usually of short duration. In many cases the pulse voltage is applied between an external trigger wire wrapped around the envelope and the electrodes; this is referred to as shunt triggering. In other applications, the lamp may be internally triggered by applying the pulse voltage directly across the electrodes, a technique referred to as injection triggering.
When the flash tube ignites upon being pulse triggered, it has been observed to inherently produce radio frequency (RF) interference from 14 KHz and 1 GHz. Such radiated RF noise is extremely undesirable in a number of applications in the broadcast band (540-1770 KHz), the VHF band (50-500 MHz) and the UHF band (300-500 MHz) where it interferes with direction finding, navigation and VHF and UHF communications equipment.
In addition to being a source of undesirable radiation, the outboard mounting of these strobe lights subjects their flash tubes to other source of radiation which is of sufficient magnitude to flash the tubes. For example, a high-powered search radar might trigger a flash tube during every revolution of the radar's antenna and thus greatly reduce the life of the tube. As these flash tubes are relatively expensive and only have an average life of about 500 hours of operation, the accidental flashing greatly increases the costs of keeping these lights in operation.
Various approaches have been made in an endeavor to reduce the electromagnetic interference caused by strobe lights. For example, in U.S. Pat. No. 3,840,766, which issued Oct. 8, 1974, to John A. Pappas and Robert J. Cosco, there is described an improved flash tube with reduced RF noise emission which is accomplished by providing a flashed barium deposit on portions of the inside surface of the flash tube.
Another method employed to reduce electromagnetic interference involves coating the strobe light lens with a thin film, such as a film made of oxides of various metals, such as gold, tin, chromium and copper. Although these films reduce electromagnetic interference, they also reduce the amount of light that passes through the lens.